A display device such as a mobile terminal, a tablet, a touch panel and PDA (Personal Digital Assistant) generally employs a tempered glass which is chemically reinforced. The tempered glass is configured by a glass base material and a surface reinforced layer (chemically reinforced layer) on its top surface. This configuration enables the tempered glass to have the thinner thickness while realizing the higher strength against bending stress and impact.
The processing of the tempered glass having the surface reinforced layer of a certain thickness or more and a certain surface compression stress or more (for example, the thickness of the surface reinforced layer is 40 μm or more, and the surface compression stress is 600 MPa or more) is not easy. Accordingly, a method of processing the tempered glass of Patent Publication 1 describes the tempered glass having the surface reinforced layer of 30 μm or less and the surface compression stress of 600 MPa or less which is processed by employing a known cutting method (such as laser beam machining). Further, Patent Publication 2 proposes a method of processing a tempered glass (the thickness of the surface reinforced layer is 40 μm or more, and the surface compression stress is 600 MPa or more) having a surface reinforced layer a part of which is removed for weakening the processing strength at an expected cutting position. Then, in this method, an expected cutting trench is formed and cut by using a laser.
However, in Patent Publication 1, only the workability of the tempered glass is attached importance and a further thinning and a further strengthening which are currently required are not satisfied by employing the method of Patent publication 1.
The formation of the expected cutting trench on the surface reinforced layer is essential in Patent publication 2 to increase the number of the steps, and further the processing on the tempered glass is restrictive because the expected cutting trench may be formed only in a linear fashion.
Under these circumstances, the present inventor firstly pays the attention to a processing method in which a processing device is vibrated and rotated and which has been recognized difficult to be applied to the processing of the tempered glass.
The product glass is specifically formed by cutting out a plurality of raw plates for final product in the shape of the product from a large substrate made of a tempered glass, and by processing the respective cut-out raw plates for final product including aperture formation processing.